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| Titel |
Pedogenic carbonate recrystallization rates and periods are regulated by temperature-dependent rhizosphere processes: Relevance for paleoenvironmental applications |
| VerfasserIn |
Martina Gocke, Yakov Kuzyakov |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250051591
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| Zusammenfassung |
| Interest in secondary (pedogenic) carbonates as an archive for paleoclimatic reconstructions
in soils and sediments of arid and semiarid climates has increased during recent decades.
Pedogenic carbonates are formed by precipitation of Ca2+ from the soil solution with
dissolved CO2 from soil air, originating mainly from root and rhizomicrobial respiration.
Therefore, the isotopic signature (δ13C, δ18O, Δ14C) of pedogenic carbonates represents the
conditions prevailing during their formation, and is widely used for reconstruction of
paleovegetation, -temperature and -precipitation, as well as for estimation of the age of
pedogenesis. However, for the interpretation of chronological and paleoenvironmental
information from pedogenic carbonates, knowledge of the time frame of pedogenic carbonate
formation is crucial. Formation rates and periods as well as the influence of climatic factors
remain unknown.
The aim of this study was to reveal the influence of growing season temperature on
pedogenic carbonate recrystallization. We hypothesized that temperature-dependent biotic
processes like plant growth and root and rhizomicrobial respiration have stronger influence
on soil CaCO3 recrystallization than abiotic temperature-dependent solubility of CO2 and
CaCO3.
In loess as a common soil parent material, initial CaCO3 recrystallization rates were
determined with the 14C isotopic exchange approach by exposing loess several times to
14CO2 released by root and rhizomicrobial respiration of plants labeled in 14CO2 atmosphere
at 10, 20 or 30 Ë C. 14C recovered in recrystallized CaCO3 was quantified to calculate
amounts of secondary CaCO3 and corresponding recrystallization rates, which were in the
range of 10-4 – 10-6 day-1. Recrystallization rates in rhizosphere were always
at least up to 2 orders of magnitude higher compared to non-rhizosphere loess,
demonstrating the importance of vegetation for pedogenic carbonate formation and
recrystallization. Moreover, strongly increasing rates with increasing temperature
showed the major role of biological activities for initial CaCO3 recrystallization
rates, especially rhizosphere processes like root and rhizomicrobial respiration and
enhanced water uptake by roots, which are boosted by increasing temperature. The
abiotic effect of lower solubility of CO2 in water by increasing temperature was
completely overcompensated by biotic processes. Based on initial recrystallization
rates, periods necessary for complete recrystallization were estimated for different
temperatures, presuming that CaCO3 recrystallization in soil takes place mainly during the
growing season. Taking into account the shortening effect of increasing temperature on
the length of growing season, the contrast between low and high temperature was
diminished, yielding recrystallization periods of 5740 years, 4330 years and 1060 years at
10, 20 and 30 Ë C, respectively. In summary, increasing CaCO3 recrystallization
rates with increasing temperature demonstrated the important role of vegetation for
pedogenic CaCO3 formation and the predominantly biotic effects of growing season
temperature.
Considering the long periods of pedogenic carbonate formation lasting to millennia, we
conclude that methodological resolution of paleoenvironmental studies based on isotope
composition of pedogenic carbonates is limited not by instrumental precision, but by the time
frame of pedogenic carbonate formation and hence cannot be better than thousands of years. |
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